This invention relates to liquid-solid separation equipment, and more particularly to liquid-solid separating centrifuge apparatus comprising a rotary bowl which is spun at a relatively high rate of speed (500 RPM or more) to effect centrifugal separation of the liquid and solid phases present in a slurry delivered to the centrifuge.
Reference is made to U.S. Pat. No. 4,381,849, disclosing prior art liquid solid separation centrifuge equipment. This equipment comprises a rotary bowl of generally cylindrical configuration having a generally conical end extension at one end thereof and a bowl-head at the other end thereof. A rotary conveyor is mounted coaxially in the bowl and a motor and gear train are provided for rotating the bowl and the conveyer about a common axis in the same direction, but at different speeds of rotation. A solid-liquid slurry is delivered to the bowl and is collected in a pool of slurry adjacent the cylindrical inner surface of the bowl. Centrifugal force causes the slurry held in the pool to separate into its liquid and solid phases with the heavier phase (i.e., the solids) migrating radially outwardly toward the surface of the bowl. The solids are transported axially along the bowl by the screw conveyor to a solids discharge port in the conical end extension. The liquid phase that has been separated from the slurry migrates to the radially inward free surface of the pool and is withdrawn via liquid transfer ports in the bowl-head. A plurality of weir plates (one for each transfer port) partially cover radially outer portions of the ports for directing the flow of overflow liquid through the radial inner portion thereof.
Inasmuch as the rotary bowl is rotating at a speed of 500 RPM or more, the overflow liquid (which thus has a linear circumferential speed of several hundred feet per second), after flowing through the transfer ports and past the weir plates, exits as a series of streams having a significant angular velocity component in the direction of rotation of the bowl. In other prior art centrifuges, an annular weir plate defining a single, continuous annular chamber is provided, such that the overflow liquid is discharged in a circumferentially continuous annular flow pattern having a significant tangential concircumferential velocity component. If the pool in the bowl becomes so deep as to cause flow of liquid through the transfer port, the flow in an annular chamber without baffling, can assume a free-vortex flow pattern, such that the tangential speed of the liquid upon discharge can exceed that of the bowl at this same radius. This phenomenon may be visualized by considering the vortex flow pattern of water created when a toilet is flushed, and the water exiting the central area of the outlet flows at an increased speed so as to conserve angular momentum. Regardless of which of these flow patterns may exist, in the prior art centrifuges, the overflow liquid discharges with significant tangential velocity and associated angular momentum which previously has been imparted within the rotating bowl, with resultant significant energy loss in the discharged overflow liquid. This lost energy becomes more significant as the speed of rotation of the centrifuge and/or the diameter of the rotary bowl is increased. For large, high speed centrifuges (especially in thickening applications) the lost energy of the overflow liquid can represent up to 80% of the power input required to operate the centrifuge.